Three-dimensional ion traps with quadrupolar fields in both the r and z (in a polar coordinate system) direction impose linear forces on ions and can be used as traps for ions of wider or narrower ranges of mass/charge values. The field shapes are usually provided by a set of three electrodes, a ring electrode and two end cap electrodes of hyperbolic shape. Such devices are known as a Paul or quadrupole ion traps. In simpler alternative devices, the cylindrical ion traps (CITs), the inner surface of the ring is cylindrical and the end caps are flat.
The Paul trap and the cylindrical ion trap have known deficiencies. They include limits on the number of ions that can be trapped and low efficiencies for external ion injection. In order to minimize space charge effects and so achieve high resolution in commercial mass spectrometers, only 500 ions or fewer can be trapped in a typical experiment. The ion population injected through the entrance hole in the end cap electrode experiences the RF fields and only those ions injected at the right RF phase can be effectively trapped. Collision with buffer gas assists in trapping and the overall trapping efficiency for ions injected continuously is less than 5%, in many cases much less.
Another class of ion traps, the linear ion traps, address these problems. A linear ion trap includes elongated spaced multiple rods with RF and DC voltages applied to trap ions in the volume defined by the multipoles. A linear ion trap with elongated multipole rod sets is described in U.S. Pat. No. 6,177,668. A two dimensional RF field radially confines those trapped ions that fall in a mass range of interest. The ions are contained axially in the volume defined by the rods by a dc field applied to the end electrodes. Trapped ions are axially and mass selectively ejected by mixing of the degrees of freedom of the ions caused by fringing fields. U.S. Pat. No. 6,403,955 is directed to a quadropole ion trap mass spectrometer in which the trapping volume is defined by spaced rods. The motion of ions in the trapping volume produces image currents characteristic of the ions. U.S. Pat. No. 5,420,425 describes a linear quadrupole ion trap in which the ions are ejected through an elongated aperture formed in one of the spaced linear rods defining the trapping volume. All of the above ion traps, except the cylindrical ion trap, require accurate mechanical processing such as machining, assembly, etc., which is further complicated when making small portable mass analyzers employing ion traps.
U.S. Pat. No. 6,483,109 discloses a multiple stage mass spectrometer. One preferred embodiment includes a pulsed ion source coupled with a linear array of mass selective ion trap devices, at least one trap being coupled to an external ion detector. Each ion trap is configured with a storing cell for ion trapping interspersed between a pair of guarding cells, all aligned along their z axis. Radio frequency (RF) and direct current (DC) voltages are applied to electrodes of the ion trap device to retain ions within the storing cells. Each trapping cell has a sub-region in which the dynamic motion of the ion exhibits m/z-dependent resonance frequencies along the z direction, allowing the ion motion to be selectively excited by m/z value. The AC voltages can be combined with time-resolved changes in the applied DC voltages to enable individual trapping cell to be switched between ion trapping, mass selecting and ion fragmenting modes. Ions may be selectively transferred between ion traps, and selectively dissociated within each trap to enable an MSn operation. The linear array of ion traps comprises harmonic linear traps (HLTs) composed of a plurality of open cells. The cells of the HLTs are composed of parallelpiped rectangular electrodes oriented in the ZX and ZY planes with no rectangular electrode in the XY plane.